During coal-based power generation in power plants flue gases form, which contain harmful nitrous gases (NOx) in addition to ash particles and other pollutants. In order to comply with statutory limit values, provision has to be made in power plant systems for corresponding measures to reduce the quantity of pollutant in the waste gas.
Thus for example in coal-fired power plants the so-called SCR (selective catalytic reduction) method is used to comply with the required NOx values. With this method a reduction product in the form of ammonia (NH3) is sprayed into the flue gas flow and the resulting mixture is then routed by way of a catalyzer. Nitrogen and steam are thus produced in the catalyzer. This catalytic reaction reduces the NOx concentration in the flue gas. In this process the NOx values are measured at emission points that have previously been officially approved in the system. The half-hour mean values and daily mean values are then calculated from these in an emission computer (EMI computer), registered and forwarded to the competent authorities for analysis.
The NOx values are frequently measured at the outlet of the catalyzer, as it is here that the best control quality can be achieved for regulation-related reasons. However the nature of the system means that the NOx values may be different at the outlet of the catalyzer (measurement point) and at the actual chimney outlet (emission values to be taken into account). Therefore not only is the measurement site different but generally also the quality of the measurement. The official measurement must be calibrated and standardized regularly, while the actual controlled variable does not have to be, which can also produce diverging results.
In order to be able to comply reliably with the emission values required by the legislator, a plant manager must therefore take this into account when setting the setpoint pollutant value, in this instance the setpoint NOx value, at the measurement point. NOx output can also fluctuate as a function of the day and load, so the plant manager will generally select quite a large safety margin between the setpoint value and the permissible limit value. In order always to operate the system within the optimum operating range, the plant manager must therefore constantly adjust the setpoint value as a function of the currently measured values. Until now this was always done manually. Non-optimum operation of the system or too large a safety margin between the set setpoint value and the legally permissible limit value also results in higher ammonia NH3 consumption.